Second Session Part 4
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35 Second Session Part 4 Before we discuss what telescope to buy and how to set it up let us discuss Standard types of Telescopes, Eyepieces and Mountings1 Of course, the next thing you want to know is how to set up your telescope so that you can find celestial objects as fast and easily as possible and track them without loosing them out of sight almost as soon as you found them. Knowledge that you gained so far, most of all knowledge of basic geometrical features of the Alt/Az – and Equatorial systems will help you greatly. However, let us pause for a while and discuss what kind of instrument you are using when you say that you are using a telescope. As an amateur astronomer you are using an instrument which “aids in the observation of remote objects by collecting electromagnetic radiation” (Wikipedia: “telescope”) - such as light. (The word “telescope” comes from the Greek “tele” - “far, distant” - and “skopein” – “to view”). Other types of electromagnetic radiation (gamma ray/x- ray/ultraviolet/infrared/microwave/radio etc.) can also be observed using telescopes especially designed for this purpose; yet, amateurs mostly contend themselves with visible light and leave the rest of the electromagnetic spectrum to the professionals. Hence, optical telescopes are our domain. But there are different kinds of optical telescopes. They are partly classified according to their optical design, partly by the kind of mounting on which they are mounted, partly by their specific purpose or way of functioning. 1 All drawings of this section are taken from F.W. Price, The Moon Observer’s Handbook, Cambridge: CUP 1988. 35 36 First of all, there are three basic optical designs of amateur telescopes: the refractor, reflector and the catadioptric. The Refractor It is commonly assumed that Jan Lippershey of Holland invented the “spy-glass” or telescope in 1607. He invented a refractor. Immediately afterwards, Galileo Galilei (1564-1642) built his own spy-glass and revolutionised our understanding of the universe with it. It had a very simple design. It was made of a convex objective lens with a focal length of roughly a meter, fixed to one end of a tube, and a concave lens – the eyepiece - at the other end. (When you stick out your tummy, it becomes convex; when you pull it in, it becomes concave; it “caves in”). The length of the tube could be adjusted for finding a reasonably sharp image. Point the telescope to an object, look through the eyepiece, adjust the length of the tube until you find a reasonably sharp image and, hey presto! Things come closer. Picture of an Amateur Refractor 36 37 Drawing of optical design of refractor However, this kind of refractor has at least two faults. The image formed by a single convex objective lens produces rainbow-coloured fringes around objects. The lens refracts light like a prism splitting white light into rainbow colours) and causes chromatic aberration. Drawing of optical path involving chromatic aberration Also, the image is never really sharp because the lens refracts light in a way that its “rays” cannot coincide in a single joint focus. This is called spherical aberration. 37 38 Drawing of spherical aberration Both faults can be almost abolished if the front lens consists of a convex and a concave lens of different glass with different refracting properties - joined together. This is a doublet. It is called an achromatic lens. The telescope is called an achromat. Most amateur refractors are achromats (or apochromats). The Reflector In the 17th century, before Dollond invented the doublet objective lens, refractors suffered from severe chromatic aberration. Isaac Newton had the idea to use a (spheroidal) concave primary mirror for creating a magnified image of a distant object. Mirrors of this kind do not have chromatic aberration. If their focal length is long enough (f/10 – 15), i.e. the distance from the mirror in which the reflected image comes to a focus, spherical aberration can also be held at a minimum. Image formation of such a mirror looks like this: However, in order to see a distant object magnified, one would have to hold one’s head near the focal point – which will obstruct the view almost entirely. We do not want our head to obstruct the view. Furthermore, we might wish to have a reasonably long focal length in order to get a decent image showing a lot of fine detail free of spherical aberration. Newton had the idea of putting a secondary (flat) mirror into the optical path. It deflects the image in such a way that the focal point lies 90 degrees off the optical 38 39 axis. A magnifying eyepiece put into the eyepiece tube allows us to look at the image produced at the focal point. An Amateur Newton reflector This is its optical path: 39 40 Even reflecting telescopes are not perfect in spite of their advantages of having far less chromatic and spherical aberration, their tremendous light gathering power, their lighter weight and shorter length than refractors of the same aperture (diameter of the front lens/mirror). However, paraboloidal mirrors, frequently used in Newtonian telescopes of shorter focal lengths suffer from another kind of aberration called coma which makes stars at the edge of the field of view look like little comets. Of course, no one wants this kind of distortion either, particularly if you wish to take wide field images of the night sky. Spheroidal concave mirrors, however, are free of coma, although they need a correcting plate, in fact a kind of specially formed lens to slightly change the direction of the rays of light falling on the mirror. Catadioptrics The third class of telescopes combines mirrors and lenses into a single optical system in the attempt to overcome all three kinds of aberrations. The two most popular telescopes of this kind are the Schmidt-Cassegrain- and the Maksutov-Cassegrain telescopes. They can have small focal ratios and, hence, wide fields of view free of coma, chromatic and spherical aberration, ideal for astrophotography. A Schmidt Cassegrain telescope A Schmidt Cassegrain has a spheroidal mirror, combined with an aspheric correcting plate (lens). It also has a small convex mirror built into the rear centre of the 40 41 correcting plate facing the primary mirror. It reflects the light back into a central hole in the primary mirror which leads into the eyepiece tube. A Maksutov telescope is very similar to a Schmidt Cassegrain. Instead of an aspheric correcting plate it has a spherical corrector which is not as thin as the one in the Schmidt Cassegrain but easier to manufacture. It is excellent, provided the glass for the corrector has no internal faults and does not absorb too much light. 41 42 Light grasp, magnification and resolving power One of the most frequently asked questions by people who look through a telescope for the first time is: ”What is its magnifying power?” They mostly assume that a telescope must be better, the higher its magnification is. Like myself when I started, you may have bought a small refractor or reflector from LIDL because you were told that it magnifies up to 200 times. Let us assume that you learnt to use it successfully to look at Saturn or Jupiter. Then you go to an observatory, look through a much larger telescope which also magnifies 200 times and you find that you see much more, more detail, more and fainter stars, even tiny and very faint galaxies. Obviously, magnification is not the major factor when it comes to assessing the quality of a telescope. The most important piece of information about the “the power” of a telescope is its aperture, the diameter of its front lens or primary mirror. Its surface area determines the telescope’s light grasp. It determines its image brightness. The larger the diameter, the more you will be able to see faint detail. Light grasp is proportional to the square of the objective/ mirror diameter. An 8 inch Schmidt Cassegrain is twice as big as a 4inch; but it has 4 times its light grasp. (Of course, the price of increasingly larger telescopes frequently increases in the same exponential way). You can understand the relation between aperture and light grasp fairly easily if you think of your own eyes and how its pupils function. On a bright summer’s day, your pupils may only open 1-2mm wide. You are almost blinded by the light. The pupil area for vision might just be 4 square millimeters. Incoming light has to be reduced in order for you to see clearly. If you suddenly step into a really dark room, you cannot see anything, of course. You are almost blinded by darkness. After some minutes, certainly after 20 minutes, your pupils will widen up to 8mm and, hence, you will see more and more again. Your pupil area for vision might increase roughly to 55 square millimeters. You are dark adapted because your light grasp was largely increased due to the increased diameter of your pupil. Now compare the pupil area for vision of roughly 55 mm square with a small LIDL refractor with an objective lens of 60 mm. The surface area is already ca. 2800 mm square. This is 56 times larger than our widest pupil area. Now compare this with the surface area of a 45 cm (16 inch) telescope. Its area for light grasp is 160 000 mm square which is 57 times the area of our 60 mm LIDL telescope. Moving from the surface area for light grasp of our pupil to a 60mm telescope is almost the same step as moving from a 60mm telescope to a 45 cm giant amateur scope! Higher light grasp is expensive, though, very expensive.